How Often Should a Roof Be Cleaned?

Why there is no single interval

Roofing trade sites quote all sorts of figures for how often a roof should be cleaned, and they rarely agree. That is because the answer is local. A tiled roof in an open estate on a hill and one in a hollow beside woodland are different environments, even if the tiles are identical. Moss follows moisture and shade, so the two will not need attention on the same schedule.

What does hold true everywhere is that growth is easier to deal with early. A light film of algae or a few small cushions of moss can be brushed away and treated quickly. A thick mat holds water against the tile surface, can lift the edges of tiles and can fill the gutter when it falls. Waiting until the roof is solid green costs more time and more money.

There is another reason to avoid a fixed rule. A roof that was cleaned and treated carefully, with the gutters left clear, can look good for a long stretch, while one that was only hosed down may green up within a couple of seasons. Two cleans of the same roof can therefore have quite different results. The method counts as much as the gap between visits.

What changes how often it is needed

Shade and trees come first. Branches overhanging a slope slow its drying and drop debris that moss roots in. Next is the covering. Concrete tiles tend to take moss more readily than slate or smooth clay, and old tiles with a worn surface take it more readily still. The pitch plays a part because shallow roofs shed water slowly. Whether a biocide was used last time also matters a great deal, which is why we describe it on the page about roof biocide treatment.

It is worth knowing what returning growth looks like. The first sign is usually a green or black bloom on the lower courses and along the shaded side. For more on where moss turns up first, see why moss grows on north-facing roofs.

Neighbouring buildings can change things too. A new extension, a tall fence or a mature hedge that has grown up since the last clean can throw a shadow across a slope that used to dry quickly. If you notice that part of the roof now stays dark long after rain, it is a good sign that the cleaning gap for that slope has shortened, even though the roof itself has not changed.

A simple routine between cleans

Once a year, usually in late autumn when the leaves are down, look over each slope from the ground and from upstairs windows. Binoculars help. Look at the line along the gutter, the valleys, the area around the chimney and any slope that stays damp. Check that the gutters are flowing, since a blocked gutter feeds the tiles above it.

Take a photo from the same spot each year. Comparing them is the easiest way to see whether growth is slow or fast, and it takes the guesswork out of deciding when to call someone. If you cut back branches, the roof usually dries out better and stays clear for longer.

Keep a note of the date of each clean and what was done to the roof, such as whether a biocide went on afterwards. When you next call someone, that note saves a lot of guessing. It also helps you spot a clean that did not last as long as it should have, which is worth raising with whoever did the work.

When to ask us to look

Call us if moss has reached the gutters, is spreading into the joints between tiles, or has lifted anything at the edge. Also call us if the roof has been cleaned several times and keeps coming back quickly, because that can mean the tiles themselves are worn and a different answer is needed. We look at the roof with a free survey and photograph it, so you can see what is going on, and the fixed written quote follows.

Typical UK figures for hand removal with biocide are about £748 for a small roof up to about £978 for a large one, and the fixed written quote only comes after we have surveyed the roof. If the roof is clean and healthy we say so, and leave it alone. Our main page on roof cleaning in Basingstoke sets out how we approach the work.